Various types of communication systems transmit signals that may radiate in a portion of the electromagnetic spectrum and cause interference with devices that operate in that portion of the electromagnetic spectrum (e.g., radio frequency (RF) spectral bands). In some cases regulatory requirements for certain geographical regions (e.g., imposed by governments) place constraints on power that may be radiated in certain spectral regions, such as amateur radio bands. Some systems are wireless systems that communicate between stations using radio waves modulated with information. Other systems are wired systems that communicate using signals transmitted over a wired medium, but the wired medium may radiate enough power in restricted spectral bands to potentially cause interference.
Communication stations can be configured to avoid using or limit the amount of power that is radiated in certain restricted spectral bands. Alternatively, communication stations can be configured to adjust the spectral regions used for communication, based on whether the station is operating in an environment in which interference may occur. For example, orthogonal frequency division multiplexing (OFDM), also known as Discrete Multi Tone (DMT), is a spread spectrum signal modulation technique in which the available bandwidth is subdivided into a number of narrowband, low data rate channels or “carriers.” To obtain high spectral efficiency, the spectra of the carriers are overlapping and orthogonal to each other. Data are transmitted in the form of symbols that have a predetermined duration and encompass some number of carriers. The data transmitted on these carriers can be modulated in amplitude and/or phase, using modulation schemes such as Binary Phase Shift Key (BPSK), Quadrature Phase Shift Key (QPSK), or m-bit Quadrature Amplitude Modulation (m-QAM). An example of a system in which carriers can be disabled to avoid potential interference is described in more detail in U.S. Pat. No. 6,278,685, incorporated herein by reference. In this system, after one or more carriers are disabled, the modulation functions (e.g., an interleaver shift mechanism) are adjusted for a different number of usable carriers.